Mounts for optical elements are basically designed depending on the requirements for imaging quality and the given conditions under which the optical system in which the mounted optical element forms a component part is to be transported, stored and used. In particular, anticipated shock loads, possible temperature fluctuations during transport, storage and use and the energy related and spectral influence of radiation during use are factors in this respect. The optical element should be permanently held in position with low strain irrespective of the aforementioned loads.
Based on the above mentioned requirements for a subassembly according to the invention, only those prior art subassemblies or mounts which, like a subassembly according to the invention, hold a rotationally symmetrical optical element in a mount so as to allow radial expansion to be compensated over a given temperature range will be considered hereinafter as component parts of such subassemblies. Subassemblies of this type are referred to as thermally compensated.
Laid Open Application DE 10 2006 060 088 A1 discloses an optical subassembly with a monolithic mount (referred to as a holder in the above-cited reference) in which a rotationally symmetrical optical element is held in a mounting ring by three connection units which are integrated in the mount. The connection units comprise in each instance bending beams (referred to as web) which tangentially contact the optical element. The optical element is held with low strain due to the radial elastic flexibility of the tangentially contacting bending beams, and different thermal expansions between the optical element and mount can be compensated. The optical element is always held so as to be centered. The bending beams contact the circumference of the optical element centrally in each instance and are connected at both ends to the mounting ring. In the basic state of the optical subassembly, the bending beams are substantially relaxed. They are radially tensioned when the optical element expands radially due to a change in temperature. A drawback of this principle of configuring the mount with bending beams tangentially contacting the optical element consists in particular in that external forces acting on the mount which do not act in radial direction on the connection between bending beams and optical element can lead to uncontrolled, irreversible shifting of the optical element, particularly to twisting around the optical axis thereof.
An optical arrangement with a monolithic mount and a rotationally symmetrical optical element which is held therein via three connection units (referred to as spring leg arrangements) is also known from DE 10 2010 008 756 A1. The spring leg arrangements are formed in each instance of two parallel spring legs, one of the ends of both parallel spring legs being connected in each instance via flexure bearings to a mounting ring (referred to as outer mount area), while the other ends open into a contact base at which the optical element is fixed by gluing or soldering. The two parallel spring legs are arranged at a distance from one another in direction of their flexibility, i.e., perpendicular to the optical axis of the optical element, this distance being small in proportion to their length, and extend along a concave curvature line as viewed from the optical element. In case of radial expansion of the optical element, the latter exerts radially acting forces on the contact bases, which leads to the deflection of the parallel spring legs in a perpendicular plane relative to the optical axis. Accordingly, by comparison with the central connection of a bending beam to the optical element according to the above-cited DE 10 2006 060 088 A1, no bending torque would occur in the region of contact with the optical element, for the reason that the spring leg arrangement would itself generate a torque in the contact region that would oppose the torque exerted by the optical element, as a result of which the contact base would only be able to execute a translational movement.
In case of differing expansion of the optical element and mount, there would only be a negligible, slight rotation of the optical element around its optical axis. Since the connection between the spring leg arrangements and the optical element is not loaded, it can advantageously be formed by a bonding connection. This principle of configuring the mount has the drawback that a connection unit designed as a spring leg arrangement of this type occupies a large amount of space so that only three can be arranged.
In both of the mounts mentioned above, the connection units are constructed as spring joints. In a perpendicular plane with respect to the axis of symmetry of the mount, in which plane the optical axis of the optical element extends in case of a centered arrangement of a rotationally symmetrical optical element, these spring joints have only a low spring stiffness so that the optical element can be held with low strain. In view of the desired low spring stiffness of the spring joints in one operative direction, the spring stiffness is also relatively slight in an adjoining area around this operative direction, which can lead in particular to twisting of the spring joints within themselves resulting in a tilting of the optical axis of the optical element with respect to the axis of symmetry of the mount.
It was also noted in both publications that the construction of the mounts as monolithic component parts is merely advantageous, and mounts based on the illustrated principles can also be realized by means of discrete components.